a ferrite bead can be thought of as a very lossy inductor, or inductor with very low q. U want such a thing in a power supply line, for instance, because u do NOT want to form banpass filters along a DC bias line. How? Well if u put high q inductors every 2" along a line....when your frequency is such that 2" is around a half wavelength, you have actually formed a bandpass response..which will readily LET the microwaves by th einductors.
If they are lossy inductors, this does not happen, because even IF there were a banpass, its loss would be 30 dB or so.
But ferrites are...just a mash of iron type junk. the composition, grain size, binding material, all causes them to work well at some frequencies, and poorly at others.
If you were interested in 4 Ghz leakage, u would want a ferrite bead that had resistive loss at 4 ghz, maybe peaking at 4.5 ghz, so u have to slog thru the manufacturers data sheets. If by ferrite bead, u mean surface mount chip....that is easier, there will be easy to read test charts on the data sheet. U will find most ferrite beads optimized for << 1` GHz